1. Field of the Invention
The invention relates to the field of virtual reality, and in particular to the rendering of sound in virtual world.
2. Art Background
Modern computer systems can manipulate many forms of information: text, graphics, video, and even sound. Sound is typically stored in a computer system in the form of digital information which can be converted to an analog form recognizable by the human ear. The analog sound is rendered in the physical world through speakers or headphones connected to the computer system, in manners well known in the art.
The volume or intensity of a sound varies according to the location of the listener relative to the origin of the sound. Sounds which originate far away from the listener tend to have less intensity than sounds which originate close by. This decrease in sound intensity with distance from the sound origin is called attenuation. Also, the intensity at which the sound is perceived by each ear on the human head varies according to the orientation of the head relative to the sound source. The variation of sound intensity with distance from the sound source and orientation of the listener is known as sound localization.
Sounds in the real world do not originate from a dimensionless point source. Rather, the source of a sound (such as a bell, for example) has physical dimensions. Within the physical dimensions of the sound source the sound intensity is constant, since it would not be possible in the real world for a listener to "enter" the physical dimensions of the sound source. This region in which the intensity of the sound is perceived as constant is known as the ambient region for the sound, and reflects the fact that sounds originate not from an infinitely small point (a point source), but from a region in space. Furthermore, sounds in the real world have direction; they travel farther in some directions than in others.
Modern computer systems are used to model the physical world through the use of virtual reality technology. Using virtual reality technology, virtual worlds may be created in which a user of the computer system is given the experience of moving through a three-dimensional model of a physical world on the output of the computer system. It is desirable to provide these computer system users with a sound experience to match the visual experience of the virtual world. To provide this sound experience, it is desirable that sounds rendered from the computer system to the users by way of speakers, headphones, or other means reflect characteristics of sounds in the real world. Sounds rendered in a virtual world should have attenuation and direction characteristics which are similar to the attenuation and direction characteristics of sounds in the physical world.
FIG. 1 shows a prior art model for describing the intensity of a sound in a virtual world. The audible zone 110 for the sound (the region in which the sound can be heard) is comprised of a sound origin 100 and a plurality of vectors 130 which define the intensity of the sound along radial vectors originating at the sound origin 100. The audible zone 110 also comprises a plurality of interpolation zones 120 comprising the areas between vectors 130.
Each vector 130 radiates outward from the sound origin 100. The intensity of the sound is localized along the length of the vector 130. Longer vectors indicate that the sound travels farther along the direction of the vector 130. Using a series of vectors to model the sound requires the computer system to store descriptions of each vector 130, which may consume a significant amount of storage space, especially in three dimensions. While interpolation techniques may be used to define the sound intensity in the interpolation zones 120, the complexity of the sound definition is increased by the use of interpolation. It would be desirable to implement a fully parametric model for defining the intensity of a sound for all points in space. A parametric model would have the advantage of being more concise than the vector-based model of FIG. 1, and would not suffer from the added complexity and the need for interpolation.
FIG. 2 shows another prior art model for describing the intensity of a sound in a virtual world, using a cone. The audible zone 210 is comprised of a sound origin 230 and a direction vector 200. The sound origin 230 is located at the point of a cone which defines the audible zone 210. The sound is directed along the axis of the cone, and radiates outward from the axis in a region defined by the volume of the cone. Points farther along the axis (or radially positioned thereabouts) typically exhibit greater sound attenuation than do points closer to the origin. Unfortunately, while a cone offers a good model for the propagation of beams of light, it is less than ideal for modeling the propagation of sound. Sound in the physical world tends to radiate outwardly from its origin in all directions, not just within the volume defined by a cone. Furthermore, although sounds in the physical world propagate in all directions from their origin, they do so asymmetrically. Sounds tend to propagate farther in one direction than in others. For example, the sound of someone's voice will normally propagate farther in the direction which they are facing than it will propagate in the direction behind them. It would be desirable to devise a model for the propagation of sound which accounts for the asymmetrical propagation characteristic of sound in the real world.
FIG. 3 shows another prior art model for describing the intensity of a sound in a virtual world. The first audible zone 310 is conical, and is comprised of a direction vector 300 which originates from a sound origin 340. The sound origin is surrounded by a second audible zone 320 which is spherical in shape. At the intersection of the first and second audible zones is a problem area 330. A sound model combining spheres and cones also has drawbacks. Such a model leads to unnatural sound behaviors, especially in the regions at or close to the problem area 330 where the spherical and cone volumes intersect. Within the problem area 330, the sound is not audible, and yet a small lateral move in either direction places the listener within either the first audible zone 310 or the second audible zone 320, where the sound is heard at or close to full intensity.
It would be desirable to define a parametric model for the intensity of sound in a virtual world which accounts for the directional and localization characteristics of sound in the physical world. The model should be simple and intuitive enough to be easily understood by persons designing virtual worlds.